Draft: lpc driver: make disp v2 brightness pwm setting available in sysfs for sysctl updates

The latch chip keeping the power on during rp2040 reset can only work if disp v2 brightness (disp v1 enable) is either fully 0 or 1 (100%). Currently some people run a buggy rp2040 firmware on Pocket that has the value locked at 80%, and they have V1 displays so they can't set the brightness to 0 in userspace yet (if V1 display is detected, brightness control is not enabled by the LPC driver). With this patch, they should be able to set it to 0 anyway via sysfs, and will be able to do a firmware update.